Radio frequency switches are used in many aspects of present day communication systems and radar systems including for example in cellular telephones and phased array radar antennas. Today, such radio frequency switching is often accomplished with the use of solid-state devices such as Field Effect Transistors and PIN diodes or with macro sized metal-to-metal contact switches or relays. Such solid state devices are often small and easily integrated with other radio frequency components but provide relative poor electrical performance. In contrast the larger in size Macro switches offer relatively good electrical performance including isolation measuring greater than 70 decibels, insertion losses near 0.07 decibels and contact resistances of less than one ohm however such switches are bulky and not easily integrated with many radio frequency components.
One solution to these difficulties is provided by the micro-sized or microelectromechanical or MEMS metal contact switch. Such MEMS switches may be fabricated using the same fabrication processes as is used in realizing solid-state devices. The size of these switches makes them easily integrated with radio frequency components and additionally, because they are mechanical devices, such switches provide relatively good radio frequency performance including isolation greater than 20 decibels and insertion losses near 0.1-0.5 decibels. Although the MEMS switches of the present invention are viewed as being primarily useful at radio frequencies the described structure and method are not limited to such usage and may indeed find application in any frequency range between direct current and signals in the gigahertz range.
Radio frequency MEMS metal contact switches have been fabricated and tested by industry, government laboratories, and academia. The upper electric contact area for previous switches has been “plug-shaped” with a flat bottom. Flat upper electric contacts are, however, not easily cleaned, have inconsistent wear patterns, and do not allow for switch operation at different areas on the contact surface.
This invention provides a way of implementing a “hemispherical-shaped” upper electric contact geometry into micro-switch fabrication and includes defining the upper contacts in a sacrificial layer using standard photolithography. The resulting electric contact geometry is then re-flowed in an oven to reform, by surface tension, the “plug-shaped” contact into a “hemispherical-shaped” contact. This allows for reliable contact cleaning (i.e. mechanical wiping) and consistent metal-to-metal contact with each switch actuation and also allows the switch to be operated at different areas on the contact surface by varying the switch actuation voltage.